The invention is directed to a stereoscopic technique for inspection of periodic structures such as large scale integration (LSI) wafers or masks.
Wafer inspection, in particular, has been a problem since a wide variety of imperfections are tolerable. Color changes due to oxide thickness variation, slight pattern misalignment, and some pattern aberrations are acceptable. Presently, wafers are inspected by human operators who use binocular microscopes to examine the structures on the wafer one at a time and then judge whether a particular defect is critical or not. Such a procedure is time consuming, tedious, and highly subjective.
In an effort to facilitate wafer or mask inspection, some partially automated techniques have been developed. However, a fully or partially automated system for wafer inspection is both expensive and technically complex at this time. Mask inspection, in which less deviation is tolerable, is more amenable to partially automated techniques. One such technique is the Automated Mask Inspection System (AMIS) which is described by Sittig et al. in Proceedings of the Kodak Microelectronics Seminar, Oct. 29-30, 1973, p. 49-52. Adjacent patterns on a mask are compared by optically scanning the corresponding parts with a pair of laser beams and subtracting the resultant output to obtain an indication of differences. An adaptation of AMIS to wafer inspection is described by Cuthbert in copending application U.S. Ser. No. 473,233, filed on May 24, 1974, now U.S. Pat. No. 3,944,369, issued Mar. 16, 1976, and assigned to the assignee hereof, in which electronic gating is added to avoid the rejection of wafers having acceptable pattern aberrations. That is, two scanning light beams are intensity modulated by patterns on the reference and test samples. The modulated optical signals are coupled into electronic circuitry which gates preselected combinations of the signals in accordance with a number of predetermined threshold levels to produce an output indicative of differences between the reference and the test samples.
Another partially automated mask inspection technique utilizes spatial filtering as disclosed by Cuthbert in U.S. Pat. No. 3,790,287, assigned to the assignee hereof. Light beams scan the surface to be inspected and a reflected portion of the beam is spatially filtered so that only that portion scattered by defects is detected beyond the spatial filter.
The three partially automated systems described above all require relatively complicated mechanical, optical, and electrical apparatus. As sophistication is increased for better adaptation to wafer inspection, the equipment will likely become even more complex.
An approach to decreasing cost and complexity is to use aid-to-operator techniques. One common technique involves a blink comparator, a device for alternately viewing two nominally identical fields so that differences between the fields will appear to blink. However, presentation of the images in alternation still requires fairly complex optical devices and electrical circuits.